Complex crop rotations improve organic nitrogen cycling

Authors: BREZA, LAUREN - University Of New Hampshire; MOOSHAMMER, MARIA - University Of California; BOWLES, TIMOTHY - University Of California; Jin, Virginia; Schmer, Marty; THOMPSON, BENNETT - University Of New Hampshire; GRANDY, STUART - University Of New Hampshire

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2022
Publication Date: 12/10/2022
Citation: Breza, L.C., Mooshammer, M., Bowles, T.M., Jin, V.L., Schmer, M.R., Thompson, B., Grandy, S.A. 2022. Complex crop rotations improve organic nitrogen cycling. Soil Biology and Biochemistry. 177. Article 108911. https://doi.org/10.1016/j.soilbio.2022.108911.
DOI: https://doi.org/10.1016/j.soilbio.2022.108911

Interpretive Summary: Using crop rotations to boost soil organic nitrogen availability and the uptake of organic N by crops could help decrease the need for synthetic fertilizer inputs. Complex crop rotations can introduce a greater diversity of food sources for soil microbial activities, enhancing organic nitrogen dynamics. In this study, we found that soil organic N cycling is stimulated by more complex crop rotations, but that adding synthetic N fertilizer can decrease these benefits. Management strategies, such as crop rotation diversity, that promote internal N cycling can decrease the dependency on external N inputs while still maintaining crop productivity and decreasing the risk of N losses into the environment.

Technical Abstract: Nitrogen (N) availability in agroecosystems is often poorly coupled to plant N uptake, leading to inefficient N fertilizer use and environmental losses. Restoring soil organic N pools and enhancing the internal recycling of N with crop rotations may help improve N use efficiency. Supplemental inorganic N fertilization is often used to optimize yields, yet it remains unknown how these fertilizer additions impact efforts to accelerate organic N cycling and availability with practices such as crop rotation. We investigated how 1) crop rotations restore organic N pools and accelerate organic N cycling, and 2) inorganic N fertilization enhances, has no effect, or suppresses soil N cycling responses to crop rotation. To test this, we measured gross protein depolymerization, amino acid consumption, ammonification, and ammonium consumption rates using 15N isotope pool dilution assays on soils collected from a long-term crop complexity experiment in Mead, NE, USA. Treatments sampled included both 0 kg and 180 kg ha-1 fertilization levels in continuous corn, corn-soybean, and corn-soybean-sorghum-oat/clover rotations. We found that high cropping complexity coupled with zero fertilization increased gross depolymerization (p = 0.018) and amino acid consumption rates (p < 0.001) relative to fertilized, monocrop plots. Gross ammonification was faster in more complex rotations, irrespective of fertilizer treatment (p < 0.001), while ammonium consumption was faster in fertilized plots across all cropping regimes (p = 0.001). We show that internal N cycling is stimulated by cropping complexity; however, N fertilization suppresses some of the benefits of temporal crop diversification. Management strategies that adopt fertilization regimes that balance mineral fertilizer application and promote internal N cycling via increased cropping complexity will promote productivity while minimizing N losses.